Starting voltage suppressor circuitry for an x-ray generator

ABSTRACT

Circuitry for eliminating or reducing overshoot of the direct current high voltage of an X-ray tube by reducing the magnitude of an initial single-phase voltage to the transformer-rectifier unit compared to the magnitude of subsequent phase voltages applied thereto during an exposure.

United States Patent James et al.

s41 STARTING VOLTAGE SUPPRESSOR 3,113,259 12/1963 Walker ..323/45 x UITFOR AN X RAY 3,295,053 12/1966 Perrins..,.; ..323/45 X GENERATOR FOREIGNPATENTS OR APPLICATIONS Inventors: J J Baltimorc;'l"led J- Euler,1,144,409 2/1963 Germany ..250/103 Ellicott City, both of Md.

- Primary Examiner-Anthony L. Birch [73] Assignee. CGR MedicalCorporation, Cheverly, Md. AnomehF- H. Henson and E. P. Klipfel [22]Filed: Sept. 24, 1969 [21] Appl.No.: 860,686

1511 ABSTRACT [52] U.S.Cl ..250/102,250/103, 315/137, ircui ry foreliminating or r d cing overshoot of the direct 315/287, 317/33 SC,323/435 S current high voltage of an X-ray tube by reducing the mag-[51] Int. Cl. ..ll05g l/l8,i-105g 1/32 nitude of an initial single-phasevoltage to the transformer- [58] Field oi Search ..315/ 137, 291, 279,287, 143; rectifier unit compared to the magnitude of subsequent phase250/102, 103; 317/33 SC; 323/73, 43.5 S, 44 voltages applied theretoduring an exposure.

[ References Cited 4 Claims, 6 Drawing Figures UNITED STATES PATENTS3,539,812 11/1970 Johnson ..250/103 58 Pl 4| 24 30 594 1 2. v i

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TIMING lo AND GATING 3 CIRCUIT P2- TRANSFORMER RECTIFIER UNIT PIS-1F|G.6. 57 i i I I0 TIMING 7 AND GATING CIRCUIT STARTING VOLTAGESUPPRESSOR CIRCUITRY FOR AN X-RAY GENERATOR BACKGROUND OF THE INVENTION1 Field of the Invention The present invention relates generally toX-ray exposure control circuitryand more particularly relates tostarting voltage suppressor circuitry for eliminating'or'reducingovershoot of the direct current high voltage applied to an X-ray tubeduring an X-ray exposure to preclude overexposed radiographs and harmfulovervoltage conditions.

2. Description of the Prior Art I Multiphase X-ray generators generallyuse a high-voltage rectified power supply which produces a directcurrent output across the X-ray tube with peak-to-peak'ripple rangingfrom to percent for different units and various loads.

The ideal voltage output across the X-ray tube is a square wave butunfortunately upon start of an exposure a voltage overshoot occursgenerallydue to a combination of starting the X-ray tube on single phasewhich gives a higher peak voltage and/or the classical overshootwhichoccurs in a'high-voltage rectifier circuit on the first pulse 'dueto leakage inductance and distributed capacity in theX-ray generator.The duration of such voltage overshoot is usually 0.003xto 0.006seconds. I

The high-voltage surge can amount to a percent overshoot. This is veryobjectionable for at least two reasons: (a) on a short time exposure of0.003 or 0.006 the X-ray exposure will be made at a higher voltage thatis set on the X-ray control and will result in overexposed radiographsand (b) the peak kilovolt settings near the top rating of the X-raygenera tor will cause overvoltage on the components of the equipment.The resulting overvoltage condition on the rectifiers, cables, X-raytubes, etc., can impair the reliability of the system or require greaterpower handling capabilities of the 4 equipment than is necessary forproper operation.

Therefore, an object of the present invention is to provide circuitryallowing accurate exposures even when of extremely short time duration.l

Another object of the present invention is to provide circuitry forpreventing overvoltaging of components within the X-ray generator whenoperated near rated voltage.

SUMMARY OF THE INVENTION Briefly, the present invention accomplishes theabove-cited objects and other objects and advantages by providingcircuitry which upon start of an exposure reduces the magnitude of thevoltage input to the high-voltage rectifier circuit during an initialphase to eliminate or hold to a negligible value overshoot of thevoltage toan X-ray tube. More particularly, the multiphase control inputto a high-voltage transformerrectifier unit feeding an X-ray tube issequenced in the same way at the. start of each exposure. The magnitudeof the voltage of the initial phase applied to the transformer-rectifierunit is reduced compared with the magnitude of subsequent phases.Thesequence is arranged to provide the desired magnitude of three-phasevoltage to the transformer-rectifier unit after the transient time ofvoltage overshoot upon-start ofan exposure has passed.

'BRIEF DESCRIPTION OF THE DRAWING Further objects and advantages of thepresent invention will be readily apparent from the following detaileddescription taken in conjunction with the drawing in which:

FIGS. 1 and 2 illustrate waveforms useful in understanding the operationof the present invention;

FIG. 3 is a schematic .diagram of an illustrative embodiment of thepresent invention;

FIG. 4 is a schematic diagram of an item utilized in the illustrativeembodiment of FIG. 3;

FIG. 5 is an electrical schematic diagram of another illustrativeembodiment of the present invention; and

FIG. 6 is an electrical schematic diagram of still another.embodiment ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The magnitude and duration ofthe high-voltage direct current across an X-ray tube determines theextent of an exposure. A multior three-phase X-ray machine generallyincludes a high-voltage rectified power supply which produces ahigh-voltage DC output across the X ray tube having a magnitude which isa function of the phase-to-phase voltage applied to the primary of ahigh-voltage transformer of the transformer-rectifier unit.

The ideal voltage output across an X-ray tube would bea square wave '2as illustrated in FIG. 1. In actual practice the output from thetransformer-rectifier unit hasa peak-to-peak ripple ranging from 5 to 20percent for different units and various loads. Such an actualvoltageoutput is as illustrated by the waveform 4 of FIG. 2. Uponinitiation of an exposure the direct current voltage across the. X-raytube generally overshoots the desired magnitude. Such overshoot can bedue to a combination of either starting the unit on single phase whichgives a higher voltage and/or the classical overshoot which occurs in ahigh-voltage rectifier circuit on the first pulse due to leakageinductance and distributed capacity in the unitJThe duration of such anovershoot is generally on the order of 0.003 to 0.006 seconds. Theduration, of course, varies with the desired magnitude of direct currentvoltage to be applied across the X-ray tube and the X-ray tube anodecurrent (load).

In some instances, it is desirable that the X-ray generator provide avery short exposure time such as in the range of the .aforementioned0.003 to 0.006 seconds. These short exposure times are desirable toavoid distortion in the radiograph as a result of motion. Thehigh-voltage surge is undesirable when very shorttime exposures are tobe taken since the X-ray exposure will be made at a higher voltage thanis set on the X-ray control and will result in overexposed radiographs.Exposures having peak kilovolt settings near the top rating of the X-raygenerator can result in overvoltage on the components of the equipment,thus impairing the reliability of the system.

FIG. 3 illustrates circuitry for preventing overshoot of kilovoltage atthe start of an X-ray exposure. Tenninals l0 providemeans for.connection of a three-phase supply to a control autotransformer l2herein illustrated with Y-connected windings l4, l6 and 18 witha neutralpath 20. Tap switches 24, 26 and 28 are gang operated by a coupling asindicated at 30. The phase-to-phase voltage output from theautotransfor'mer l2 is'as determined-by controlling the positioning ofthe tap switches. j

The transformer rectifierunit-40.connects the phase-tophase voltageoutput from the control autotransformer to an X-ray tube 41.

Switching means illustrated as silicon-controlled rectifiers connect theoutput control voltages from the autotransformer 12 to thetransformer-rectifier unit 40. Silicon-controlled rectifiers 51, 52.and53 are located in connecting lines and .poled in the samedirection.Silicon-controlled rectifiers 54,

55 and 56 are connected in the same lines but poled in the oppositedirection from their companion silicon-controlled rectifier.

A schematic diagram of the transformer-rectifier unit 40 is asillustrated in FIG. 4. A high-voltage. transformer 47 is connected tothe control autotransformer 12 via its primary windings 42 at theprimary input terminals P1, P2 and P3. A Y-connected secondary 43 anddelta connected secondary 44 are connected to rectifier bridges 45 and46 to provide a highvoltage direct current output'across the X-ray tube41.

Referring again to FIG. 3 data accumulated from many oscillograms onvarious multiphase X-ray generators indicate that the degree ofovershoot is proportional to the peak kilovolt setting on the X-raycontroLThe peak kilovolt output across the transformer-rectifier unit tothe X-ray tube 41 is a function of the phase-to-phase voltage applied tothe primary 42 of the high-voltage transformer 41 in the unit 40.

The desiredvoltageselected at the control autotransformer 12 isconnected to the transformer-rectifier unit 40 by enabling signals froma timing and gating circuit 57 which gates selected SCR switches in acontrolled sequence.

The main primary contacting or switching is controlled by SCR switches51 through 56 which, as shown in FIG. 3 are silicon-controlledrectifiers. These could be replaced by mechanical contactors. The SCRswitches are turned on in a controlled sequence in the same way at thestart of each exposure. For example, switches 51 and 56 are gated ONfirst applying single-phase voltage across terminals P1 and P3 in theunit 40. Then 60 to 90 electrical degrees later switch 52 is gated ONfollowed by switches 53, 54 and 55 in the proper sequence to completethe application of three-phase voltage.

The present invention avoids voltage overshoot on the output byproviding an auxiliary autotransformer 58 connected across tap switch 24and neutral 20. A silicon-controlled rectifier 59 connects a portion ofthe auxiliary autotransformer 58 to the input terminal P1 of thetransformer-rectifier unit 40.

In such a manner a lower voltage than that set on the controlautotransformer 12 is supplied to the primary 42 of the high-voltagetransformer 41 during the period that singlephase voltage is applied atthe start of the exposure.

When the SCR switches are turned on in a controlled sequence in the sameway at the start of each exposure the reduced magnitude of the voltageof the initial phase upon start of an exposure will eliminate or reducethe overshoot to negligible values. Then normal voltage of the magnitudeselected at the control autotransformer 12 is applied with thesubsequent phases.

The voltage tapped off auxiliary autotransformer 58 may be, for example,approximately 20 percent lower than the voltage at the tap switch 24.The reduced magnitude of voltage is applied to the input terminal 51through silicon-controlled rectifier 59. The timing and SCR gatingcircuit 57 is arranged so that at the start of an exposure, single-phasevoltage of a reduced value is applied to input terminals P1 and P3through the gating of switches 59 and 56.

After 90 electrical degrees and before switch 54 is gated on, switch 59is gated off. On succeeding in pulses of the proper polarity switch 51is gated on instead of the switch 59 thus applying the full voltage setat the tap switch 24. The auxiliary autotransformer 58 could also havethe same effect when connected across tap switch 28 and neutral 20 ortap switch 26 and neutral 20. When desirable, the auxiliaryautotransformer 58 could also be connected from tap switch to tapswitch. In each case the starting sequence would have to be alteredaccordingly.

An alternate embodiment of the present invention is illustrated in FIG.5 wherein like items have been identified with identical referencecharacters used in FIG. 3. Referring to FIG. 5, an extra tap switch 25is mechanically coupled to tap switch 24, replacing the auxiliaryautotransforrner 58, so that a fixed reduced voltage proportional to thepeak kilovolt setting on the controlled autotransformer 12 is appliedthrough switch 59. Alternatively, voltages could be tapped from theprimary windings 42 associated with the input P1 to the high-voltagetransformer 41 in the unit 40. Thus the auxiliary autotransformer 58 ofFIG. 3 would not be required at all.

Still another illustrative embodiment is as illustrated in FIG. 6 whereonce again like items have been identified with identical referencecharacters. A bucking transformer 60 is connected with one winding 61 inseries with the switch 59 and input terminal P1 with another winding 62connected across tap switch 24 and neutral 20. The primary winding 62could also be connected across other legs of the autotransformer 12 in asimilar manner or across tap switches. The transformer 60 injects abucking voltage which reduces the applied voltage from tap switch 24during the first impulse upon start of an exposure.

While the present invention has been described to the degree ofparticularity for purposes of illustration, it is to be understood thatall modifications, alterations and substitutions within the spirit andscope of the present invention are herein to be included. For example,although silicon-controlled rectifiers have been illustrated as themeans for switching it is to be understood that bidirectionalsemiconductor switches and even mechanical contractors may be utilized.

We claim as our invention: 1. In a multiphase X-ray generator, thecombination comprising:

an X-ray tube; transformer-rectifier means coupled to said X-ray tube,

being responsive to the magnitude of a multiphase alternating currentinput voltage for providing a direct current voltage to said X-ray tube;three-phase control autotransformer means coupled between saidmultiphase input voltage and said transformer-rectifier means forcontrolling the magnitude of the multiphase input voltage applied tosaid transformerrectifier means for a desired exposure; auxiliaryautotransformer means connected across one phase of said controlautotransformer means for reducing the magnitude of the voltage inputfor an initial phase upon start of an exposure; and auxiliary switchingmeans connecting said auxiliary autotransformer means to saidtransformer-rectifier means for applying at the start of an exposure asingle-phase voltage thereto a reduced value compared to the magnitudeof subsequent phase-to-phase voltages applied thereto whereby overshootof the voltage to said X-ray tube is eliminated or held to a negligiblevalue. 2. In a multiphase X-ray generator, the combination comprising:

an X-ray tube; transformer-rectifier means coupled to said X-ray tube,

being responsive to the magnitude of a multiphase alternating currentinput voltage for providing a direct current voltage to said X-ray tube.three-phase control autotransformer means including a tap switchconnected to each phase and ganged together, coupled between saidmultiphase input voltage and said transformer-rectifier means, thesetting of said tap switches controlling the magnitude of thephase-to-phase alternating current voltage applied thereto for a desiredX-ray exposure; and another tap switch connected to one phase of saidcontrol autotransformer, coupled to said transformer-rectifier meansproviding a fixed reduced voltage thereto compared to the voltagesetting of said tap switches for an initial phase upon start of anexposure whereby overshoot of the voltage to said X-ray tube iseliminated or held to a negligible value. 3. In a multiphase X-raygenerator, the combination, comprising:

an X-ray tube; transformer-rectifier means coupled to said X-ray tube,

being responsive to the magnitude of a multiphase alternating currentinput voltage for providing a direct current voltage to said X-ray tube;control autotransformer means coupled between said multiphasealternating current input voltage and said transfonner-rectifier meansfor controlling the magnitude of the voltage applied to saidtransformer-rectifier means; and bucking transformer means including afirst and a second winding, said first winding being connected to saidcontrol autotransformer means and said second winding being connected tosaid transformer-rectifier means so as to buck the voltage of theinitial phase of said input voltage from said control autotransforrnermeans upon start of an exposure whereby the input voltage applied tosaid transformer-rectifier means is reduced in magnitude compared to themagnitude of subsequent phases whereby overshoot of the direct currentvoltage to said X-ray tube is eliminated or held to a negligible value.4. In a multiphase X-ray generator, the combination comprising:

an X-ray tube;

transformerrectifier means coupled to said X-ray tube, being responsiveto the magnitude of a multiphase AC input voltage applied thereto forproviding a DC voltage to said X-ray tube;

three-phase control autotransformer means coupled between saidmultiphase AC input voltage and said transformer-rectifier means forcontrolling the magnitude of the multiphase voltage input appliedthereto for a desired exposure;

switching means interconnected between said autotransformer means andsaid transformer-rectifier means for selectively connecting each phaseof said AC input voltage to said transformer-rectifier means;

means coupled to one phase of said autotransfonner means for reducingthe magnitude of the voltage for an initial phase of said AC inputvoltage;

timing and gating means coupled to said switching means, gating saidswitching means to connect the phases of said AC input voltage to saidtransformer-rectifier means in a controlled sequence in the same way atthe start of each exposure; and

said switching means further including an auxiliary switch connectingsaid means for reducing the magnitude of the voltage for the initialphase to said transformer-rectifier means, said timing and gating meansadditionally being coupled to and enabling said. auxiliary switch onlyduring v the period that said initial phase of said AC input voltage isapplied at the start of any exposure whereby overshoot of the voltage tosaid X-ray tube is substantially eliminated or held to a negligiblevalue.

1. In a multiphase X-ray generator, the combination comprising: an X-raytube; transformer-rectifier means coupled to said X-ray tube, beingresponsive to the magnitude of a multiphase alternating current inputvoltage for providing a direct current voltage to said Xray tube;three-phase control autotransformer means coupled between saidmultiphase input voltage and said transformer-rectifier means forcontrolling the magnitude of the multiphase input voltage applied tosaid transformer-rectifier means for a desired exposure; auxiliaryautotransformer means connected across one phase of said controlautotransformer means for reducing the magnitude of the voltage inputfor an initial phase upon start of an exposure; and auxiliary switchingmeans connecting said auxiliary autotransformer means to saidtransformer-rectifier means for applying at the start of an exposure asingle-phase voltage thereto a reduced value compared to the magnitudeof subsequent phase-to-phase voltages applied thereto whereby overshootof the voltage to said X-ray tube is eliminated or held to a negligiblevalue.
 2. In a multiphase X-ray generator, the combination comprising:an X-ray tube; transformer-rectifier means coupled to said X-ray tube,being responsive to the magnitude of a multiphase alternating currentinput voltage for providing a direct current voltage to said X-ray tube.three-phase control autotransformer means including a tap switchconnected to each phase and ganged together, coupled between saidmultiphase input voltage and said transformer-rectifier means, thesetting of said tap switches controlling the magnitude of thephase-to-phase alternating current voltage applied thereto for a desiredX-ray exposure; and another tap switch connected to one phase of saidcontrol autotransformer, coupled to said transformer-rectifier meansproviding a fixed reduced voltage thereto compared to the voltagesetting of said tap switches for an initial phase upon start of anexposure whereby overshoot of the voltage to said X-ray tube iseliminated or held to a negligible value.
 3. In a multiphase X-raygenerator, the combination, comprising: an X-ray tube;transformer-rectifier means coupled to said X-ray tube, being responsiveto the magnitude of a multiphase alternating current input vOltage forproviding a direct current voltage to said X-ray tube; controlautotransformer means coupled between said multiphase alternatingcurrent input voltage and said transformer-rectifier means forcontrolling the magnitude of the voltage applied to saidtransformer-rectifier means; and bucking transformer means including afirst and a second winding, said first winding being connected to saidcontrol autotransformer means and said second winding being connected tosaid transformer-rectifier means so as to buck the voltage of theinitial phase of said input voltage from said control autotransformermeans upon start of an exposure whereby the input voltage applied tosaid transformer-rectifier means is reduced in magnitude compared to themagnitude of subsequent phases whereby overshoot of the direct currentvoltage to said X-ray tube is eliminated or held to a negligible value.4. In a multiphase X-ray generator, the combination comprising: an X-raytube; transformer-rectifier means coupled to said X-ray tube, beingresponsive to the magnitude of a multiphase AC input voltage appliedthereto for providing a DC voltage to said X-ray tube; three-phasecontrol autotransformer means coupled between said multiphase AC inputvoltage and said transformer-rectifier means for controlling themagnitude of the multiphase voltage input applied thereto for a desiredexposure; switching means interconnected between said autotransformermeans and said transformer-rectifier means for selectively connectingeach phase of said AC input voltage to said transformer-rectifier means;means coupled to one phase of said autotransformer means for reducingthe magnitude of the voltage for an initial phase of said AC inputvoltage; timing and gating means coupled to said switching means, gatingsaid switching means to connect the phases of said AC input voltage tosaid transformer-rectifier means in a controlled sequence in the sameway at the start of each exposure; and said switching means furtherincluding an auxiliary switch connecting said means for reducing themagnitude of the voltage for the initial phase to saidtransformer-rectifier means, said timing and gating means additionallybeing coupled to and enabling said auxiliary switch only during theperiod that said initial phase of said AC input voltage is applied atthe start of any exposure whereby overshoot of the voltage to said X-raytube is substantially eliminated or held to a negligible value.